Insulated-gate (MOS) type semiconductor devices, such as IGBT (insulated gate bipolar transistor) and power MOSFET (power metal-oxide semiconductor field-effect transistor) are widely used for power device which control large electric power.
A configuration of a general IGBT is explained here by referring to FIG. 1. A collector electrode is connected to a p-type collector layer. An n-type base layer is fabricated on the collector layer. In the upper region of the n-type base layer, a p-type body region is fabricated. Within the body region, an n-type emitter layer is fabricated. A region of the n-type base layer configured in between two emitter layer is a channel region. On top of the channel region, a gate insulating film and a gate electrode are fabricated. On the emitter layer, an emitter electrode is fabricated. These n-type and p-type regions may be fabricated by including elements such as P (phosphorus) and B (boron) in the substrate comprising Si and other material. The n-type and p-type regions may alternatively fabricated by ion-implantation of P, As (arsenic), and B at certain dose amount, acceleration voltage, and implantation angle prescribed for each of the regions, followed by activation heat treatment at temperatures and durations predetermined for each of the regions.
Metal wires and ribbons which are connected with external terminal are connected to upper surface of the emitter electrodes of IGBT. Collector electrodes are directly fixed and connected to circuit board through soldering layer.
In an IGBT with a p-type channel region, an inversion layer is formed in the channel region by applying positive bias to the gate electrode while applying negative and positive bias onto emitter electrode and backside electrode, respectively. An emitter layer and an n-type base layer are electrically connected with each other through the inversion layer. As a result, an electrical current drifts to the collector electrode.
Al-based films such as, for example, pure Al and Al—Si alloy are used for the emitter electrode.
In the process of fabricating the IGBT, ion implantation is carried out to form a collector layer from the back side of the substrate after fabricating an emitter electrode. Subsequently, activation of the implanted ions is promoted by carrying out a heat treatment at 450° C. or lower. Thermal stress by such heat treatment is thus induced to the aforementioned emitter electrode and other portions of the device. Thermal stress is further induced to the aforementioned electrode and other portions of the device in actual operation environment for IGBT which is repeatedly subjected to a temperature range from approximately 250° C. to 350° C.
In cases such as the temperature of the heat treatment exceeds 450° C. in the device fabrication process, and the device is repeatedly subjected to a temperature range from 250° C. to 350° C., the Al-based film constituting the electrode is deteriorated by the formation of so-called hillock which is unusual geometry of surface projection, anomalous deposition of added alloy elements, and inter-diffusion of atoms across the interface of adjacent films. Therefore, the heat treatment temperature has been restricted to 450° C. or lower. The device has also been compelled to be operated at low temperatures.
Patent literature 1, for example, describes a heat treatment at 800° C. to 950° C. for the purpose of activation of the collector layer. The heat treatment at such high temperatures, however, is restricted before fabrication of an interconnection. Once the interconnection is fabricated, the heat treatment is carried out up to about 450° C., and no description is found in the literature referring to heat treatment at higher temperatures. The heat resistance in an actual operation environment is not evaluated at all.